Questions tagged [luminosity]

Luminosity is the energy radiated from a body per second.

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How to convert contrast ratio to number of stops?

I have read an article from the below link about the human eye sensitivity and came to know that its dynamic contrast range is 1000000:1 or in other words 30 stops. They also mentioned static contrast ...
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35 views

Luminosity of the sun

How is it that the total power generated by nuclear reaction in the core of Sun exceeds its total luminosity? Where does the difference go?
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27 views

Magnetic flux density compard to other definition of flux

With regards to flux i have always assumed it was defined as $$(1) Something/m^2s$$ For instance $Luminosity/(4\pi r^2)$ Is an example of radiation flux. Why then is faradys law often cited as the ...
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21 views

Luminostiy, Flux, Intensity definitions and meaning

I am taking astrophysics course and we are constantly talking about Luminosity,Brightness,Intensity,Flux etc. I thought that I understand the idea properly but I see now that I dont. Intensity is the ...
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96 views

Why is the relation between luminosity distance $d_L$ and comoving distance $\chi$ $d_L=\chi/a$?

The textbook (Scott Dodelson, Modern Cosmology, Section 2.2 Distance, Page 35-36) states as follows: Another way of inferring distances in astronomy is to measure the flux from an object of known ...
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94 views

Luminosity of a white dwarf

Why in an H-R diagram the white dwarfs are at the bottom left? I understand that they would have a high temperature when they are formed, and so at that instant shouldn't they have a high luminosity ...
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2answers
57 views

Finding the distance to a distant star using standard candles

I understand how we can measure the luminosity of a close star through absolute and apparent magnitude, because we can easily find the distance through trigonometric parallax. What I have understood ...
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1answer
125 views

What is the closest distance for an undiscovered old neutron star and how long would it take to get here? To help people scared of neutron stars

This is to help people who are scared of neutron stars. My questions are What is the faintest a neutron star could be How close could it be and not yet be detected How long would it take to get here ...
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26 views

Stephan Boltzmann law

I was preparing for INAO when I came across the application of Stephan Boltzmann law...it's says that energy absorbed per unit time by a planet is Flux times πr^2. But why did we multiply with πr^2 or ...
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29 views

Do bigger stars emit higher amplitude waves?

So I’ve been reading about luminosity and I get the concept of the size and temperature determining the intrinsic brightness and total energy emission of the star. I also understand how the further ...
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48 views

Where can I find values for apparent brightness of stars?

I'm in high school and doing a project. I want to calculate the distance to stars using their luminosity and apparent brightness, from the equation $b=\frac{L}{4 \pi d^2}$. I have found values for ...
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100 views

How to calculate luminosity in g-band from absolute AB magnitude and luminosity distance?

How can I calculate the (non-bolometric) luminosity $L$ of a galaxy (or a star for that matter) over a given band from its AB apparent magnitude $m_{AB}$ over that band and its luminosity distance $...
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187 views

Mass Luminosity Relation Review

I have read that exists a relation between the luminosity and the mass of a star $$\frac{L}{L_\odot}=\left(\frac{M}{M_\odot}\right)^a$$ with $a$ depending on the mass $M$. I have looked for a ...
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1answer
776 views

Are lumens linearly additive?

If I have two light sources side by side emitting $1000~\rm lumens$ each, am I going to have the same amount of light emitted in an enclosed space (e.g. a room) as if I had a single light source that'...
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1answer
86 views

Illuminance at a given distance from a diffuse emitting light box

Therapeutic light boxes purport to deliver 10,000 lux to a user, typically sitting ~24" from a box. I was wondering how many lumens are necessary to achieve this illuminance, e.g. what lamps would be ...
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37 views

Safe Laser displays

I hope this is in the right stack exchange. Here's my question, assuming that all blue lasers are too risky to the eye to risk [using], is it possible to use only low-energy red and green lasers and: ...
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1answer
397 views

Maximum light intensity?

From what I had learned and observed, every image (light per certain area,let it be cm^2 here) can be projected onto a small or large area without any natural obstruction from intensity. So, if light ...
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2answers
1k views

How the Hertzsprung-Russell Diagram allows us to calculate distance to stars

I understand how to interpret a H-R diagram, in the sense that I know that the upper right top corner is occupied by cool stars, but they are very luminous so they must be big; and the bottom left ...
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2answers
265 views

How to calculate energy per range of nm

$K=hc/\mathrm{wavelength}$ is the usual for finding energy for one particular wavelength. But how would one calculate a range of wavelengths incident on a particular sized surface area? Because no ...
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37 views

Object that is visible only at certain light intensity

I have a lux meter, by which we can measure light intensity. We have one box, in which we inject the light from a hole. What we want is, if light intensity for e.g. Lux value is between 400 to 450 ...
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1answer
653 views

Converting lux to watts with known wavelength

I was wondering if there was a possible way to convert lux to watts accurately if the wavelength is known. I know that for the sun there is an approximate conversion of 0.0079W/m$^2$ per Lux using ...
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2answers
243 views

How would a nuclear explosion look in vacuum?

Approximately, how does the specific luminosity, $L_\nu(t)$, of a nuclear explosion in a vacuum look as a function of frequency and more importantly, time? Assume the observer is very far away. If ...
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1answer
141 views

Schechter' luminosity function

I need your help. I need show that the Schechter' Luminosity Function with: $\alpha = -1.5$ the limited-magnitude of a survey will be detected from a sample of galaxies with this characteristics: ...
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2answers
171 views

Relation between baryonic mass and luminosity [duplicate]

I have one simple question: How do we know that dark matter is not just ordinary matter which is simply not luminous? For example if we would take a black rock and put it somewhere in the universe, ...
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90 views

Stars with same colour

This is the problem I'm trying to solve. When it says they have the same colour, does that mean that they have the same temperature?
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968 views

Relationship between Stefan-Boltzmann Law and Radiance

In a textbook, the equation for radiance is given I am struggling to understand the relationship between the two equations. From what I know, multiplying the value obtained by the Stefan-Boltzmann ...
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What is “white light” ? Uniform wavelengths or uniform frequencies ?

Suppose you have a mixture of electromagnetic waves of wavelengths spreaded on the visible spectrum only (from $\lambda_{\text{min}} \sim 400 \, \text{nm}$ to $\lambda_{\text{max}} \sim 700 \, \text{...
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106 views

How to define the light “color” from a given spectral distribution?

The following question may be naive and incomplete in some way I don't know. I'm not a specialist on spectroscopy, colours and light curves, color spaces, etc. Suppose you have a simple power-law ...
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1answer
587 views

Radiation pressure in terms of luminosity?

My class slides have given me the following equation for $P_{rad}$: $P_{rad}=\frac{4\sigma T^4}{3c}$, where $\sigma$ is the Stefan-Boltzmann Constant. I know that radiation pressure is pressure on a ...
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175 views

Distribution of wavelengths of monochromatic light sources in expanding FLRW universe

I'm looking for the distribution of all wavelengths (or frequencies) of light that a stationary observer would receive at his location (at $r = 0$ and time $t_0$), from light sources emitting a single ...
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1answer
117 views

Can the luminosity distance be greater than the particle horizon distance?

While writing my notes on cosmology, I found this little puzzle, which isn't described in any of my books on general relativity. Consider a flat space ($k = 0$) filled with dust-like matter, of ...
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1answer
487 views

Olbers' paradox in a closed space

I'm looking for a proper interpretation of the following, which is a variant of the Olbers' paradox. Consider a spatially closed empty universe ($k = 1$) with a positive cosmological constant $\...
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39 views

what is the telescopic retinal illuminance of a star?

What is the retinal illuminance (in trolands) or luminance (in $cd/m^2$) of a star image viewed through a telescope? I'm looking for a formula that will calculate this value for input values of star ...
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54 views

Number of stars with apparent flux

The stars in our Galaxy have luminosities ranging from $L_{\text{min}}$ to $L_{\text{max}}$. Suppose that the number of stars per unit volume with luminosities in the range of $L$, $L+dL$ is $n(L)dL$. ...
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2answers
349 views

Redshift Versus Luminosity

I understand that there is a relation between the proper distance of a cosmic object and its "measurable" redshift, i.e. once you know the value of the redshift parameter z, then you actually know how ...
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56 views

Interpreting LED output distribution specification

I have specifications for an LED which gives the following graph of spacial output distribution. I am not sure how to interpret this. As I see it, it could be one of two ways. To make explanations ...
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1answer
117 views

What will happen in this LASER setup?

If I have 3 rectangular mirrors, say of dimensions length of 1 meter and width of 0.5 meter and I arrange them in the form of an equilateral triangle, which is of side 0.5 meter.(The mirrors are ...
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0answers
42 views

Where does this Gamma formula come from?

This is the derivation of formula of Gamma Correction for CRT monitors. I am not finding this in any book or web-site. Books seem to derive this formula in different ways. Can anyone please tell me ...
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1answer
699 views

Convert from Relative Magnitude to Mass

I have data which gives me the magnitude density (${\rm mag}\,{\rm arcsec}^{-2}$) of M31 as a function of radius. How can I convert these data to the (enclosed) mass at a given radius (for velocity ...
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47 views

Lorentz factor correction for luminosity

I am looking at the physics behind gamma ray bursts, working through a comprehensive review Kumar & Zhang (2014) (arXiv link). On page 92, Eq (111) the luminosity or a relativistically expanding ...
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1answer
252 views

Distance and luminosity distance

In my cosmology lecture notes I read that a way to measure distances in cosmology is to use standard candles and the comparison between "absolute luminosity" of the candle and the apparent luminosity. ...
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2answers
1k views

Explain relationship between angular diameter distance and luminosity distance, Etherington Theorem

I have a question relating to the Etherington Theorem. The luminosity distance is defined by the equation for flux, i.e. $F=\frac{L}{4\pi D_L^2}$ where flux is in units energy per unit time (...
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1answer
99 views

Why do we ignore the Flux - Luminosity relation when deriving the Eddington Luminosity?

I was just doing the derivation for the Eddington Luminosity by considering the momentum equation, $$ -\frac{Gm}{4\pi r^4} = \frac{dP}{dm} $$ and the radiative transport pressure $$ \frac{dT}{dr} = ...
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1answer
196 views

Eclipse of the sun - Intriguing luminosity curve

I've just recorded the luminosity during the sun eclipse, here is the resulting curve : Only green curve is relevant, blue curve should be ignored (it's actually the temperature). The sky was ...
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1answer
302 views

Can incandescent light bulb can be brighter if fixed power?

I found that luminous efficacy of 60 W tungsten incandescent bulb is 14.5 lm/W. Is this value defined by design details listed above or it depends on something else too? Can we design 60 W tungsten ...
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1answer
8k views

Relation between number of photons and energy?

Please can anyone explain it. If number of photons are increased will it increase the providing energy. Suppose, we are sending a limited number of photons each carrying energy. We have a energy ...
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0answers
38 views

Why don't electrons return to their ground state immediately after photoexcitation? [duplicate]

In terms of photoluminescence, why don't the electrons, which have been excited by photons earlier, immediately fall down to their ground state and reemit a photon? In other words, why does 'glow-in-...
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2answers
4k views

Position of Neutron Stars in H R diagrams

Why is that neutron stars are never depicted in a Hertzsprung-Russell diagram? They can be placed in the bottom left corner but you will never find any diagram in literature showing neutron stars.
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106 views

Is there uncertainity of position of the perfectly homogenous radiating body?

I heard the standard interpretation of Heisenberg Uncertainty Principle: Just the measurement affects the position of the body because always you want to see a body (=to measure the position), you ...
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2answers
825 views

Luminance from multiple light sources

I have a LED diode: Given Luminance intensity [I] of $1.85$ cd. I want to have Luminance [B] from the light source of $75 \ {\rm cd/m^2}$. From $B=I/S$, where S is light source plane area, I get $S=...